netns: ip6mr: declare ip6mr /proc/net entries per-namespace

Declare IPv6 multicast forwarding /proc/net entries per-namespace:
/proc/net/ip6_mr_vif
/proc/net/ip6_mr_cache

Changelog
=========
V2:
* In routine ipmr_mfc_seq_idx(), only match entries belonging to current
  netns in mfc_unres_queue list.

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>